Signal-Amplified Near-Infrared Ratiometric Electrochemiluminescence Aptasensor Based on Multiple Quenching and Enhancement Effect of Graphene/Gold Nanorods/G-Quadruplex

Particle Size-Controlled Oxygen Reduction and Evolution Reaction Nanocatalysts Regulate Ru(bpy)32+'s Dual-potential Electrochemiluminescence for Sandwich Immunoassay

Multiple signal strategies remarkably improve the accuracy and efficiency of electrochemiluminescence (ECL) immunoassays, but the lack of potential-resolved luminophore pairs and chemical cross talk hinders their development. In this study, we synthesized a series of gold nanoparticles (AuNPs)/reduced graphene oxide (Au/rGO) composites as adjustable oxygen reduction reaction and oxygen evolution reaction catalysts to promote and modulate tris(2,2'-bipyridine) ruthenium(II) (Ru(bpy)₃²⁺)'s multisignal luminescence. With the increase in the diameter of AuNPs (3 to 30 nm), their ability to promote Ru(bpy)₃²⁺'s anodic ECL was first impaired and then strengthened, and cathodic ECL was first enhanced and then weakened. Au/rGOs with medium-small and medium-large AuNP diameters remarkably increased Ru(bpy)₃²⁺'s cathodic and anodic luminescence, respectively. Notably, the stimulation effects of Au/rGOs were superior to those of most existing Ru(bpy)₃²⁺ co-reactants. Moreover, we proposed a novel ratiometric immunosensor construction strategy using Ru(bpy)₃²⁺'s luminescence promoter rather than luminophores as tags of antibodies to achieve signal resolution. This method avoids signal cross talk between luminophores and their respective co-reactants, which achieved a good linear range of 10^-7 to 10^-1 ng/ml and a limit of detection of 0.33 fg/ml for detecting carcinoembryonic antigen. This study addresses the previous scarcity of the macromolecular co-reactants of Ru(bpy)₃²⁺, broadening its application in biomaterial detection. Furthermore, the systematic clarification of the detailed mechanisms for converting the potential-resolved luminescence of Ru(bpy)₃²⁺ could facilitate an in-depth understanding of the ECL process and should inspire new designs of Ru(bpy)₃²⁺ luminescence enhancers or applications of Au/rGOs to other luminophores. This work removes some impediments to the development of multisignal ECL biodetection systems and provides vitality into their widespread applications.

Visual/Photoelectrochemical Off-On Sensor Based on Cu/Mn Double-Doped CeO2 and Branched Sheet Embedded Cu2O/CuO Nanocubes

An integrated dual-signal bioassay was devised to fulfil thrombin (TB) ultrasensitive detection by integrating visualization with the photoelectrochemical technique based on G-quadruplex/hemin. During the process, branched sheet embedded copper-based oxides prepared with illumination and alkaline condition play a vital role in obtaining the desirable photocurrent. The switchover of photoelectrochemical signal was realized by the adjustable distance between electron acceptor G-quadruplex/hemin and interface materials due to dissociation of the Cu/Mn double-doped cerium dioxide (CuMn@CeO₂)/DNA caused by the addition of TB. Then, CuMn@CeO₂ transferred onto visual zones triggered catalytic reactions under the existence of 3,3',5,5'-tetramethylbenzidine and hydrogen peroxide, making a variation in color recognized by the naked eye and providing visual prediction. Under optimized conditions, this bioassay protocol demonstrated wide linear ranges (0.0001-50 nM), high selectivity, stability, and reproducibility. More importantly, the proposed visual/photoelectrochemical transduction mechanism platform exhibits a lower background signal and more reliable detection results, which also offers an effective way for detecting other proteins and nucleic acids.

A Sensitive Electrochemiluminescence Urea Sensor for Dynamic Monitoring of Urea Transport in Living Cells

A multiple signal-amplified electrochemiluminescence (ECL) urea sensor was designed based on a self-enhanced probe and SiO₂ photonic crystals for dynamic tracking of urea transmembrane transport. The self-enhanced probe (AuNR@Ru-LA) prepared by loading polyethyleneimine (PEI), lactobionic acid (LA), and Ru(dcbpy)₃²⁺ on gold nanorods (AuNRs) generated an initial ECL signal, and then the intensity was multiple-amplified by the enhanced light-scattering effect of SiO₂ photonic crystals and the co-reaction with urea. The as-prepared sensor exhibited excellent performance for the detection of urea in the range of 1.0 × 10^-10 to 1.0 × 10^-4 M with a detection limit of 8.8 × 10^-11 M at (3σ)/S. The AuNR@Ru-LA probes were labeled on HepG2 cells to construct a cytosensor with a detection range of 1.0 × 10³ to 2.0 × 10⁶ cells mL^-1. In addition, the dynamic changes of the extracellular urea concentration were tracked by monitoring the ECL signal of the cytosensor to study urea transmembrane transport. The developed strategy realized the amplification of multiple ECL signals and the tracking of urea transmembrane transport, which provided a novel dynamic detection method for small biomolecules.

A highly selective and sensitive competitive aptasensor capable of quantifying thrombin elevated with side effects in severe COVID-19

To protect critical injury from blood clots with side effects in severe COVID-19, a highly selective and sensitive biosensor was developed for the quantification of trace levels of thrombin using the combination of a DNA aptamer (TBA) of thrombin and a complementary strand of TBA. TBA rapidly binds with thrombin, whereas it slowly binds with the complementary strand to form a double stranded DNA (dsDNA). SFC green intercalated into dsDNA cannot emit light in 1,1′-oxalyldiimidazole chemiluminescence (ODI-CL) reaction because high-energy intermediates formed from ODI-CL reaction cannot transfer energy to SFC trapped in dsDNA. However, SFC freely existing with the formation of G-quadruplex from the reaction of thrombin and TBA emits bright chemiluminescence because the high-energy intermediates can transfer energy to SFC (or camel) in solution. Thus, the brightness of light emitted in ODI-CL reaction was proportionally enhanced with the increase of thrombin in a sample due to the increase of G-quadruplex and reduction of dsDNA. The limit of detection (LOD) of the label free aptasensor operated with good linear calibration curve (10–320 mU/ml) was as low as 3 mU/ml (or 43 pM). Also, the biosensor was quantified trace levels of thrombin with good accuracy, precision, and reliability.

A ratiometric fluorescence sensor based on carbon quantum dots realized the quantitative and visual detection of Hg2

In this paper, based on the fluorescence of carbon quantum dots (CQDs) quenched by mercury ions (Hg²⁺ ) and the nonresponse of Hg²⁺ to rhodamine B fluorescence, a dual emission ratio fluorescence sensor was constructed to realize the quantitative detection of Hg²⁺ . Under excitation at 365 nm, the fluorescence spectrum showed double emission peaks at 437 nm and 590 nm, corresponding to the fluorescence emissions of CQDs and rhodamine B, respectively. This method quantitatively detected Hg²⁺ based on the linear relationship between the ratio of the intensities of the two emission peaks F₄₃₇ /F₅₉₀ and the concentration of Hg²⁺ . The detection range was 10-70 nM, and the limit of detection (S/N = 3) was 3.3 nM. In addition, this method could also realize the qualitative and semiquantitative detection of Hg²⁺ according to the fluorescence colour change of the probe under ultraviolet light. After various evaluations, the method could be successfully applied to the quantitative and visual detection of Hg²⁺ in tap water, and demonstrated excellent selectivity, anti-interference performance, and repeatability of the method.

Ultrasensitive ratiometric electrochemiluminescence for detecting atxA mRNA using luminol-encapsulated liposome as effectively amplified signal labels

It is an advantageous way to quickly identify the toxicity of Bacillus anthracis (B. anthracis) by detecting the transcription product of the atxA gene. Herein, a novel ultrasensitive ratiometric electrochemiluminescence (ECL) biosensor with competitive mechanism and double amplified signal ways was proposed for detecting the atxA mRNA. The K2S2O8 was used as cathodic emitter and silver metal-organic gels (AgMOG) was used as ECL enhancer. The AgMOG could accelerate the electro-catalytic reduction of S2O82- to SO4˙-, which reacted with dissolved oxygen, resulting in strong cathodic ECL. Meanwhile, luminol was encapsulated in liposome as anodic amplified signal labels and the luminol anion radical also reacted with dissolved oxygen to create the anodic ECL emission. We immobilized luminol-encapsulated liposomes on the surface of AgMOG through the hybridization of DNA and mRNA. This would provide a competitive mechanism involving dissolved oxygen between K2S2O8 and luminol. Benefiting from the competitive mechanism and amplified signal ways, this ratiometric biosensor achieved a wide linear relationship range from 10 to 300 fM with a low limit of detection (8.13 fM). Considering the accessible operation, favorable performance, and high universality of this strategy, this work may be used to analyze other mRNAs of bacteria.

Recent advances of functional nucleic acids-based electrochemiluminescent sensing

Electroluminescence (ECL) has been used in extensive applications ranging from bioanalysis to clinical diagnosis owing to its simple device requirement, low background, high sensitivity, and wide dynamic range. Nucleic acid is a significant theme in ECL bioanalysis. The inherent versatile selective molecular recognition of nucleic acids and their programmable self-assembly make it desirable for the robust construction of nanostructures. Benefiting from their unique structures and physiochemical properties, ECL biosensing based on nucleic acids has experienced rapid growth. This review focuses on recent applications of nucleic acids in ECL sensing systems, particularly concerning the employment of nucleic acids as molecular recognition elements, signal amplification units, and sensing interface schemes. In the end, an outlook of nucleic acid-based ECL biosensing will be provided for future developments and directions. We envision that nucleic acids, which act as an essential component for both bioanalysis and clinical diagnosis, will provide a new thinking model and driving force for developing next-generation sensing systems.

Bipolar electrode ratiometric electrochemiluminescence biosensing analysis based on boron nitride quantum dots and biological release system

In this article, we developed a novel ECL ratiometry on a closed bipolar electrode (BPE) for the sensitively and accurately detection of miRNA-21. High quantum yield and low toxicity BNQDs was synthesized and coated at BPE cathode as an ECL emitter, while the anode of BPE was calibrated via another ECL material, Ir(df-ppy)₂(pic) (Firpic). The electron neutrality at both ends of the BPE electrically coupled the reactions on each pole of the BPE. Therefore, one electrochemical sensing reaction could be quantified at one end of the BPE. By the hybridization of target miRNA-21 and hairpin, the glucose blocked in MSNs by the hairpin was released and reacted with glucose oxidase (GOD) to generate H₂O₂, thereby reducing the ECL signal of the cathode BNQDs/K₂S₂O₈ system and promoting ECL signal of anode Firpic/TPrA. Further, the G-quadruplex formed by unreacted hairpin bases consumed H₂O₂, which not only recovered the ECL of BNQDs, but also further improved the ECL emission of Firpic. Therefore, the concentration of miRNA-21 could be measured by the ECL ratio of BNQDs and Firpic. The data showed that the detection limit was 10^-15 M (S/N = 3) with the linear range of 10^-15 M to 10^-9 M. The strategy of the BPE-ECL ratio method based on BNQDs showed a good prospect in clinical application.

Anodic near-infrared electrochemiluminescence from Cu-doped CdTe quantum dots for tetracycline detection

A sensitive anodic near-infrared electrochemiluminescence (ECL) immunosensor for the detection of tetracycline, based on Cu-doped CdTe quantum dots, was fabricated for the first time in this work. We have synthesized Cu-doped CdTe quantum dots by co-precipitation. The emission spectrum of the Cu-doped CdTe quantum dots could reach the near-infrared region at 730 nm in a short reflux time. More importantly, the ECL intensity of the CdTe quantum dots was enhanced by 2 fold after Cu element doping, which was attributed to the Cu d-orbital mixed with the conduction band and valence band of the host CdTe quantum dots. Inspired by the strong anodic ECL intensity of Cu-doped CdTe quantum dots, the anodic near infrared ECL sensor was constructed to detect tetracycline by competitive immunoassay. The detection range of the developed biosensor was 0.01-10 ng mL-1 and the detection limit was 0.0030 ng mL-1. In addition, the biosensor showed outstanding selectivity, long-term stability and high reproducibility, which has great potential in the field of analysis and detection.

Sensitive and Programmable "Signal-Off" Electrochemiluminescence Sensing Platform Based on Cascade Amplification and Multiple Quenching Mechanisms

A versatile and sensitive quantum dot (QD)-based "signal-off" electrochemiluminescence (ECL) sensing system was constructed using target-initiated dual Mg²⁺-dependent DNAzyme (MNAzyme) recycling and catalytic hairpin assembly (CHA) amplification strategies. After the cascade amplification, numerous ferrocene-labeled Y-shaped DNA complexes generated on the QD-modified electrode surface. In the presence of hemin, moreover, the terminal sequence of the formed complex could assemble into hemin/G-quadruplex. Therefore, the highly efficient ECL quenching was achieved due to the multiple quenching mechanisms, including electron/energy transfer between ferrocene and QDs, the steric hindrance effects, and the horseradish peroxidase-mimicking activity of hemin/G-quadruplex. Furthermore, owing to the flexibility in regulating the recognition sequences of MNAzyme, the assaying targets can be programmed. Based on the cascade amplification and multiple ECL quenching mechanisms, the developed programmable "signal-off" ECL sensing platform demonstrates excellent sensitivity and the detection limits of 35.00 aM, 3.71 fM, and 0.28 pM (S/N = 3) for target DNA, aptamer substrate (ATP as a model), and ion (Ag⁺ as a model), respectively.

Amplified Electrochemical Hydrogen Peroxide Sensing Based on Cu-Porphyrin Metal-Organic Framework Nanofilm and G-Quadruplex-Hemin DNAzyme

A novel electrochemical hydrogen peroxide (H₂O₂) sensor based on Cu-porphyrin(Cu-TCPP)/G-quadruplex-hemin nanocomposite was constructed by assembling two-dimensional Cu-TCPP metal-organic framework (MOF) nanofilm and G-quadruplex-hemin DNAzyme. The Cu-TCPP synthesized by the surfactant-assisted method has a wrinkled two-dimensional nanofilm morphology, which gives it a large surface area and accessible active sites. Cu-TCPP exhibits peroxidase activity and good stability and can catalyze the reduction of H₂O₂. In addition, Cu-TCPP can be used as a nanocarrier for G-quadruplex-hemin DNAzyme with strong peroxidase activity to achieve "biological barcode" amplification and improve stability. The cooperative interaction of Cu-TCPP and G-quadruplex-hemin DNAzyme effectively amplifies the electrochemical response signal. Electrochemical studies have shown that the constructed sensor exhibits good electrochemical sensing performance with three linear ranges: 0.08 μM to 0.11 mM, 0.11-0.91 mM, and 0.91-8.1 mM, with sensitivities of 2315.86, 301.00, and 65.71 μA/(mM cm²), respectively, and the detection limit was 0.03 μM. In addition, the sensor shows good selectivity. In summary, this study provides a simple and effective new strategy for electrochemical sensing based on two-dimensional MOFs and artificial enzymes.

Industry Development of Derivative Functionalized Gold Nanomaterials and Their Application in Chemiluminescence Bioanalysis: Based on the Industrial Practice of China's Central Yunnan Urban Agglomeration

Electrochemiluminescence biosensor is an analytical method combining electrochemiluminescence technology with biosensor. Using nanomaterials as electrochemical luminescence sensor platform can not only immobilize a large number of biomolecules but also improve the performance of the sensor to realize the supersensitive detection of biomacromolecules. Although these methods have high sensitivity for bioanalysis, there are still some shortcomings which limit the practical application. Therefore, this paper discusses the development of functional gold nanomaterials industry and its application in chemiluminescence bioanalysis. In this paper, two methods of synthesizing luminescent functional gold nanomaterials at room temperature were studied by using chemiluminescent reagents as reducing agent and protective agent. Based on luminescent functionalized gold nanoparticles, immunoassay and DNA bioanalysis probes were constructed, and their applications in chemiluminescence and electrochemiluminescence bioanalysis were discussed. Finally, the simulation results show that the relative deviation between the experimental results and the existing clinical methods is less than 17%. The sensor has good stability and selectivity and can be used for the determination of CEA in human serum. The gold nanomaterials synthesized by further research have excellent chemiluminescence activity and can be used to label biomolecules and prepare biological probes. This article aims to explore the application of chemical methods in the transformation of new industries, to achieve breakthroughs in new products in industrial innovation, and to achieve the cross-fusion of management science and engineering disciplines and chemical disciplines. The industrial development of derivative functionalized gold nanomaterials has broad application prospects in biological analysis.

Ratiometric Electrochemiluminescent/Electrochemical Strategy for Sensitive Detection of MicroRNA Based on Duplex-Specific Nuclease and Multilayer Circuit of Catalytic Hairpin Assembly

In this study, we proposed a ratiometric electrochemiluminescent (ECL)/electrochemical (EC) biosensor based on duplex-specific nuclease (DSN)-assisted target recycling and multilayer catalytic hairpin assembly (CHA) amplification cascades for the detection of microRNA (miRNA). The DSN-assisted target recycling transformed miRNAs into a large number of ssDNA, which then catalyzed a multilayer CHA amplification cascade to produce numerous long dsDNA duplexes H_n/H_n+1 (n = 2, 4, 6, ...). Then the H_n/H_n+1 displaced the ferrocene (Fc)-labeled ssDNA (S_x+1, x = 1, 3, 5, ...) to hybridize with the S_x sequence on the gold electrode surface. Consequently, a great number of long S_x/H_n/H_n+1 duplexes were immobilized for binding Ru(phen)₃²⁺ to obtain an amplified ECL signal. Meanwhile, the EC signal of Fc was reduced, and the quenching effect of Fc to ECL signal also decreased. By measuring the ratio of the ECL signal of Ru(phen)₃²⁺ to the EC signal of Fc, quantitative analysis of miRNA-499 with high accuracy and reproducibility was obtained. The ratiometric biosensor shows high sensitivity and a wide linear range of 6 orders of magnitude. With the help of DSN-assisted target recycling, this strategy can be easily extended to detect other miRNAs without redesigning the CHA cascade system. The proposed "hybrid" ratiometric ECL/EC strategy enriches the ratiometric sensors and can find extensive applications in bioanalysis, especially for multiplex detection.

Metal-organic frameworks-based sensitive electrochemiluminescence biosensing

Metal-organic frameworks (MOFs) as porous materials have attracted much attention in various fields such as gas storage, catalysis, separation, and nanomedical engineering. However, their applications in electrochemiluminescence (ECL) biosensing are limited due to the poor conductivity, lack of modification sites, low stability and specificity, and weak biocompatibility. Integrating the functional materials into MOF structures endows MOF composites with improved conductivity and stability and facilitates the design of ECL sensors with multifunctional MOFs, which are potentially advantageous over their individual components. This review summarizes the strategies for designing ECL-active MOF composites including using luminophore as a ligand, in situ encapsulation of luminophore within the framework, and post-synthetic modification. As-prepared MOF composites can serve as innovative emitters, luminophore carriers, electrode modification materials and co-reaction accelerators in ECL biosensors. The sensing applications of ECl-active MOF composites in the past five years are highlighted including immunoassays, genosensors, and small molecule detection. Finally, the prospects and challenges associated with MOF composites and their related materials for ECL biosensing are tentatively proposed.

A wavelength-resolved electrochemiluminescence resonance energy transfer ratiometric immunosensor for detection of cardiac troponin I

In this study, a wavelength-resolved electrochemiluminescence resonance energy transfer (ECL-RET) ratiometric immunosensor from Au nanoparticle functionalized graphite-like carbon nitride nanosheets (Au-g-C3N4) to Au nanoclusters (Au NCs) has been constructed for the first time. At a working voltage of 0 to -1.2 V, Au-g-C3N4 showed a strong cathodic ECL emission with a peak at 460 nm, which overlapped well with the absorption spectra of Au NCs thus stimulating the fluorescence emission of Au NCs at 610 nm. Moreover, within this voltage range, the Au NCs showed no ECL signal; therefore, they would not interfere with the detection of the system. We used cardiac troponin I (cTnI) as an analytical model to construct a sandwich immunosensor based on the ECL-RET ratiometric strategy. By measuring the responses of the ECL460 nm/FL610 nm ratio at different cTnI concentrations, the sensitive detection of cTnI with a wide range of 50 fg mL-1 to 50 ng mL-1 and a low detection limit of 9.73 fg mL-1 can be achieved. This work enriches the wavelength-resolved ECL-RET system and provides an innovative reference for the development of more efficient and sensitive ECL-RET ratiometry.

Electrochemiluminescence methods using CdS quantum dots in aptamer-based thrombin biosensors: a comparative study

The detection of thrombin by using CdS nanocrystals (CdS NCs), gold nanoparticles (AuNPs) and luminol is investigated in this work. Thrombin is detected by three methods. One is called the quenching method. It is based on the quenching effect of AuNPs on the yellow fluorescence of CdS NCs (with excitation/emission wavelengths of 355/550 nm) when placed adjacent to CdS NCs. The second method (called amplification method) is based on an amplification mechanism in which the plasmonics on the AuNPs enhance the emission of CdS NCs through distance related Förster resonance energy transfer (FRET). The third method is ratiometric and based on the emission by two luminophores, viz. CdS NCs and luminol. In this method, by increasing the concentration of thrombin, the intensity of CdS NCs decreases, while that of luminol increases. The results showed that ratiometric method was most sensitive (with an LOD of 500 fg.mL^-1), followed by the amplification method (6.5 pg.mL^-1) and the quenching method (92 pg.mL^-1). Hence, the latter is less useful. Graphical abstract Schematic representation of three different methods (quenching, amplification and ratiometric) were applied for detection of thrombin via aptasensor. The CdS nanocrystals, streptavidin (Str) coated AuNPs and also Str-luminol coated AuNPs were used for the construction steps of the electrochemiluminescence (ECL)-based biosensor.

Gold nanoclusters-poly(9,9-dioctylfluorenyl-2,7-diyl) dots@zeolitic imidazolate framework-8 (ZIF-8) nanohybrid based probe for ratiometric analysis of dopamine

Ratiometric analysis of dopamine (DA) in complex biological system is urgently desired. In this work, a novel dual-emission fluorescence probe was fabricated by embedding both gold nanoclusters (AuNCs) and poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) dots into zeolitic imidazolate framework-8 (ZIF-8) (abbreviated as ZIF-8@AuNCs-PFO) and applied to ratiometric analysis of DA. Remarkably, encapsulating AuNCs and PFO dots into ZIF-8 not only achieved an excellent aggregation induced emission (AIE) enhancement effect on AuNCs (5 times increase), but also brought about an unique DA-triggered asynchronous fluorescence changes of AuNCs and PFO dots. The as-prepared probe exhibited excellent performance toward DA in the concentrations range from 0.01 to 10000 μmol L^-1 and good selectivity over interfering substances. The detection limit of DA was as low as 4.8 nmol L^-1. Furthermore, good stability and practicability of the probe in human serum samples suggesting its great potential for diagnostic purposes. Moreover, the quenching mechanism of AuNCs was intensively studied and summarized as three synergistic processes: (i) electron transfer between AuNCs and DA; (ii) DA-triggered architecture change of ZIF-8; (iii) fluorescence resonance energy transfer (FRET) between AuNCs and polydopamine (PDA), which offered an important theory for ZIF-based fluorescent probes.

Dual amplification ratiometric biosensor based on a DNA tetrahedron nanostructure and hybridization chain reaction for the ultrasensitive detection of microRNA-133a

A novel ratiometric electrochemiluminescence (ECL)-electrochemical (EC) hybrid biosensor with a high accuracy and reproducibility was fabricated for the ultrasensitive detection of miRNA-133a. With the help of a DNA tetrahedron nanostructure and hybridization chain reaction dual amplification strategy, the detection limit of 12.17 aM for miRNA-133a was obtained.

In Situ Growing Triethanolamine-Functionalized Metal-Organic Frameworks on Two-Dimensional Carbon Nanosheets for Electrochemiluminescent Immunoassay

A facile strategy for in situ growing triethanolamine (TEOA)-functionalized metal-organic framework (TEOA@MOF) on the two-dimensional graphene oxide (GO) or g-C₃N₄ nanosheets via the self-assembly technique was introduced. In this method, Zn²⁺ was first attached on the carbon nanosheets by electrostatic interaction; then, trimesic acid (H₃btc) acted as the complex agent and TEOA as a base for the deprotonation of H₃btc and a template, which leads to in situ growing the MOF on the carbon nanosheets obtaining a sandwich-like structure. Different types of surface analysis techniques were employed to characterize the GO-TEOA@MOFs and g-C₃N₄-TEOA@MOFs nanomaterials fabricated. The GO-TEOA@MOFs or g-C₃N₄-TEOA@MOFs nanomaterial-modified electrode brings out obviously enhanced electrochemiluminescence (ECL) behaviors due to numerous TEOA in the framework structures. Specifically, both TEOA and GO can serve as the co-reactants for the ECL system of Ru(bpy)₃²⁺ and have the synergic effect of enhancing the signal. Based on the GO-TEOA@MOFs modified electrodes, we developed a sensitive and rapid label-free ECL immunoassay strategy for human copeptin, and the linear range was 5 pg mL^-1 to 500 ng mL^-1 as well as the limit of detection was 360 fg mL^-1. This work exhibits excellent specificity and good stability of the prepared immunosensor in the practical sample determination, demonstrating it can serve as a very promising method for the clinical diagnostics of acute myocardial infarction disease.

Label-Free Ratiometric Electrochemiluminescence Aptasensor Based on Nanographene Oxide Wrapped Titanium Dioxide Nanoparticles with Potential-Resolved Electrochemiluminescence

A new "one-pot" hydrothermal method was developed for the preparation of electrochemiluminescence (ECL) nanoluminophores nanographene oxide wrapping titanium dioxide (nGO@TiO₂ NLPs). The characterization demonstrated that nGO@TiO₂ NLPs possessed a core-shell-like shape. The nGO@TiO₂ NLPs exhibited potential-resolved ECL property in neutral aqueous solution using K₂S₂O₈ as a coreactant. On this basis, a label-free ratiometric ECL aptasensor was designed. nGO@TiO₂ NLPs were used to fabricate the ECL interface for target recognition, potential-resolved ECL signal generation, and amplification. In the presence of cardiac troponin I (cTnI), the aptamer resides from the electrode surface owing to its rigidity, resulting in a reduction in charge transfer resistance of the modified working electrode and a ratio enhancement of two ECL signals of nGO@TiO₂ NLPs. According to the increased ECL ratio, cTnI could be determined by the ratiometric ECL aptasensor, with a linear dynamic range of 1.0 × 10^-13-1.0 × 10^-10 mol/L and a detection limit of 4.0 × 10^-14 mol/L, which is superior to most reported electrochemical methods. This label-free ratiometric ECL strategy with self-calibrating ability and accurate, ultrasensitive, rapid, specific analytical performance showed great promise in biosensing and clinical diagnosis. The developed strategy might extend for the sensing of other protein biomarkers by using corresponding antibodies or aptamers as recognition elements.

Near-infrared carbon dots-based fluorescence turn on aptasensor for determination of carcinoembryonic antigen in pleural effusion

Early detection of carcinoembryonic antigen (CEA) is of great significance for the screening, diagnosis, monitoring and prognosis analysis of lung cancer. Herein, a novel fluorescence aptasensor with high signal-noise ratio (SNR) was constructed to achieve highly-sensitive detection of CEA relied upon the fluorescence resonance energy transfer (FRET) between near-infrared carbon dots (NIR-CDs) and gold nanorods (AuNRs). Initially, AuNRs@SiO₂-Aptamer and NIR-CDs-DNA probe were prepared via the covalent bonding reaction between their corresponding carboxyl and amino groups, respectively. After DNA hybridization, the aptasensor was formed, meanwhile, the fluorescence of NIR-CDs was quenched by AuNRs@SiO₂. Once CEA encountered the aptasensor, it would selectively combine with CEA aptamer to unwind the preformed DNA double-strand architecture thereby resulting in the NIR-CDs-DNA detach from the surface of AuNRs@SiO₂. The attendant fluorescence recovery of NIR-CDs was linearly correlated with the concentration of CEA. According to this relationship, the NIR-CDs based "turn on" sensing system was constructed and exhibited prominent responses toward CEA in the concentration range of 0.1-5000 pg/mL and a relatively low detection limit (0.02 pg/mL). Moreover, it displayed high specificity against other biomarkers or proteins, good reproducibility and acceptable accuracy regarding human pleural effusion samples.

Recent advances of ratiometric electrochemiluminescence biosensors

Ratiometric electrochemiluminescence (ECL) assays have been widely applied in biosensing because of eliminated outside interferences and improved reliability in detection.

A dual-channel homogeneous aptasensor combining colorimetric with electrochemical strategy for thrombin

In this protocol, a dual-channel homogeneous aptasenor was proposed for protein molecule determination, employing thrombin as target analyte. The colorimetric and electrochemical transducers were combined in a single analytical system for signal readout. In this dual-channel sensing strategy, the G-quadruplex sequence was released and incorporated with hemin to form DNAzyme for naked-eye colorimetric detection. Meanwhile, the hydroxyapatite nanoparticle as signal probe was combined with magnetic nanoparticles to construct sandwich-type structure for generating the electrochemical current when thrombin was present in solution. By introducing two kinds of reporter probes and transducers, this dual-channel sensor produced two different kinds of signal to improve the analytical accuracy and diversity. The results revealed that the dual-channel sensor achieved the quantatitive determination of thrombin with low limit of detection (0.40 fM) and wide range (0.1 fM to 1 nM), which offer a promise for rapid and accurate detection of biomolecule.

Target-triggered signal-on ratiometric electrochemiluminescence sensing of PSA based on MOF/Au/G-quadruplex

Signal-amplified ratiometric electrochemiluminescence (Sa-RECL) provides an attractive approach to maximize signal-to-noise ratio through enhancing signals and eliminate interferences. In this work, we prepared a novel metal-organic framework (MOF)/Au/G-quadruplex as both quenchers and enhancers to fabricate a target-triggered ratiometric ECL sensor for high sensitive and accurate detection of prostatic specific antigen (PSA). The ratiometric ECL sensor using the dual-potential-dependent ECL emitters (quantum dots (QDs) and luminol) and MOF/Au/G-quadruplex not only achieved signal self-calibration but also realized cooperative amplification. After the sequential hybridization among of complementary DNA-QDs, PSA aptamer and pDNA-Au-Hemin-MIL-DNAzyme and the further competition of PSA, the pDNA-Au-Hemin-MIL-DNAzyme probe would keep away from the electrode surface, causing a switchover of their ECL signals from "off-on" state to "on-off" state. The ratiometric ECL aptasensor exhibits high-sensitive and accurate analytical performance toward PSA with a linear detection range from 0.5 to 500 ng mL^-1 and a detection limit of 0.058 ng mL^-1 (S/N = 3). The novel ratiometric ECL biosensor has been successfully applied to determine different serum samples of PSA, indicating its potential application in the clinical diagnosis.

Dual-signal ratiometric electrochemiluminescence assay for detecting the activity of human methyltransferase

A ratiometric electrochemiluminescence assay using CdS:Eu NCs and luminol as signal emitters was fabricated for detecting the human methyltransferase activity.

Electrochemiluminescence biosensing based on different modes of switching signals

Electrochemiluminescence (ECL) has attracted much attention in various fields of analysis owing to low background signals, high sensitivity, and excellent controllability.

Switchable electrochemiluminescence aptasensor coupled with resonance energy transfer for selective attomolar detection of Hg2+ via CdTe@CdS/dendrimer probe and Au nanoparticle quencher

In the present study, an ultrasensitive electrochemiluminescence (ECL) aptasensing assay for selective detection of Hg²⁺ was designed. In this electrochemiluminescence resonance energy transfer (ECL-RET) approach, Fe₃O₄@SiO₂/dendrimers/QDs exhibited amplified ECL emissions (switch "on" state) and with the hybridization between T-rich ssDNA(S₁) immobilized on the Fe₃O₄@SiO₂/dendrimers/QDs and AuNPs modified with complementary aptamer (AuNPs-S₂), the ECL of QDs nanocomposites was efficiently quenched (switch "off" state). In the presence of Hg²⁺ ions, formation of strong and stable T-Hg²⁺-T complex led to the release of the AuNPs-S₂ from double-stranded DNA(dsDNA) and the recovery of the ECL signal of QDs (second signal switch "on" state). Under optimal conditions, Hg²⁺ can be detected in a wide linear range from 20aM to 2µM with a very low detection limit of 2aM. The proposed ECL aptasensor showed high selectivity for Hg²⁺ determination compared to other environmentally relevant metal ions at concentration ratio more than 1000 times. The aptasensor was used for detection Hg²⁺ ions from samples of tap waters, carp and saltwater fishes with satisfactory results. The aptasensor exhibited high sensitivity, wide linear response (11 orders of magnitude), excellent reproducibility and stability. The proposed aptasensor will be a promising candidate for facile and rapid determination of Hg^2＋in environmental and fishery samples.

Highly sensitive electrochemical thrombin aptasensor based on peptide-enhanced electrocatalysis of hemin/G-quadruplex and nanocomposite as nanocarrier

In this work, we first conjugated a short peptide to thrombin binding aptamer (TBA) and bond hemin to the hybrid, effectively rendering hemin/G4-peptide more active over the original hemin/G4, so that a highly sensitive electrochemical thrombin (TB) aptasensor was developed based on it and PtNTs@rGO nanocomposite. It was the first report on the application of hemin/G4-peptide in electrochemical aptasensor. PtNTs@rGO with large surface area served as excellent nanocarrier for high loading of hemin/G4-peptide hybrids, resulting in the formation of hemin/G4-peptide-PtNTs@rGO bioconjugate as the secondary aptamer and further signal enhancement. The specific affinity of aptamer for target TB made the secondary aptamer go into the sensing interface, and then a noticeable current signal was obtained from hemin without additional redox mediators. Due to the collaborative electrocatalysis of hemin/G4-peptide and PtNTs toward H₂O₂, which was formed in situ during the process of hemin/G4-peptide-catalyzed oxidation of NADH with dissolved O₂, the current intensity increased dramatically. Such an electrochemical aptasensing system could be used to detect TB with a linear range of 0.05 pM-60nM and very lower detection limit of 15fM. Notably, this method exhibited a higher sensitivity than that of many hemin/G4-based electrochemical strategies for TB detection due to the improvement of the catalytic activity of hemin/G4-peptide. The present works opened a new way for expanding the application of hemin/G4 in biological detection. With the mediator-free, proteinous enzyme-free yet high-sensitivity advantages, this electrochemical aptasensor held great promise for other biomarker detections in clinical diagnostics.